1. Field of the Invention
The present invention relates to a plasma display panel (hereinafter, referred to as PDP) used as a display device, and more particularly to a protective film for electrodes.
2. Description of Prior Art
PDP is a display device provided with a number of small discharge spaces enclosed between two glass substrates. On a matrix display type PDP, there are provided a number of electrodes in the form of a lattice, and an image is displayed by selectively causing discharge cells on the intersection of each electrode to emit light. On a typical AC-type PDP of a surface discharge type, the display electrodes on the front panel are covered with a dielectric layer and a protective film is formed over the dielectric layer. The dielectric layer is provided so as to store the electric charge resulting from applying voltage to the electrodes, and the protective film is provided so as to protect the dielectric layer from damage by collision of ions in the discharge gas and also to emit secondary electron to decrease the discharge starting voltage. PDPs like the above are described in, for example, xe2x80x9cPlasma Displayxe2x80x9d (The Journal of the Institute of Electrical Engineers of Japan, Vol. 119, No. 6, pp 346-349, 1999).
Conventionally, a magnesium oxide film of some several hundred nm thick, formed by a film forming method such as vapor deposition, has been mostly used as the protective film. This magnesium oxide film normally has adsorbed moisture, carbon dioxide, oxygen, hydrogen, and some others. It is of a concern that these materials affect the discharge characteristic in the beginning and also have an adverse effect on the operating condition of a PDP as they are discharged as impurity gas into the charged gas during the operation of the PDP. In particular, they have an adverse effect on the secondary electron emission characteristic that significantly affects the discharge voltage.
In the production process of present PDP, panels are subjected to degassing before charging with discharge gas. Any gas that has not been removed through this degassing process remains as impurity gas in the finished product. Moisture and carbon dioxide adsorbed in the protective film are particularly hard to be eliminated, hence requiring the degassing process at a higher temperature for a longer time. In many cases, this long-time degassing process can be a decisive factor of the speed of a whole production line. Since degassing at a high temperature affects other components, there is a certain limitation.
The protective film of an AC-type PDP is required to have higher secondary electron emission characteristic and, at the same time, is stable during the operation. While, in the production process of PDP, gas components, particularly moisture and carbon dioxide, adsorbed in the protective film are eliminated so as to activate the protective film, this elimination must be accomplished easily. There has been a problem with a conventional protective film that, because the film adsorbs a great amount of moisture and carbon dioxide, much of them remains even after vacuum heating at 350xc2x0 C. As a result, they have an adverse effect on the effective secondary electron emission characteristic of the finished panels and degrade the discharge characteristic. In addition, because impurity gas is emitted from the protective film during the operation, there arises a defect that the discharge characteristic is not stable. For this reason, special measures such as increasing the heating temperature and extending the degassing time are required, resulting in an increase of the production cost.
The present invention is proposed for solving the aforementioned problems associated with the prior art, and an object of the present invention is to provide a PDP equipped with a PDP-electrode protective film that has less moisture and carbon dioxide adsorption, high secondary electron emission characteristic and superior stability.
According to the present invention that solves the aforementioned problems, there is provided a plasma display panel, having a front panel equipped with display electrodes and a rear panel equipped with address electrodes, and displaying an image by causing discharge in a discharge space formed between the front and rear panels, wherein a protective film installed on the discharge side of the front panel comprises two layers, one of which is an upper (outer) layer being made of material with high discharge characteristic and the other is a lower (inner) layer being made of material with low water-adsorption characteristic.
Otherwise, the front panel is equipped with a protective film comprising two layers, upper layer and lower layer, of different specific surface area per unit weight, which upper layer is so formed as to have a large specific surface area and a thin film thickness and which lower layer is so formed as to have a smaller specific surface area and a thicker film thickness than that of the upper layer. The upper layer is formed into a layer of material with a specific surface area of 20 m2/g or more, as converted per 1 g of the protective film, and a film thickness of 1 xcexcm or less, and the lower layer is formed into a layer of material with a specific surface area of 10 m2/g or less, as converted per 1 g of the protective film, and a film thickness of 1 xcexcm or more.
Use of a material with greater specific surface area as the protective film of PDP increases the discharge characteristic. For this reason, an oxide film with the specific surface area of 20 m2/g or more is used as the upper film. With this, the coefficient of secondary electron emission from the protective film improves, resulting in a decrease of the charge starting voltage of the PDP. When the specific surface area is smaller, on the other hand, even if adsorption of moisture and carbon dioxide is caused in the course of the film forming process or in any PDP production process after that, the absolute amount of adsorption is small and they can be eliminated easily in the heat degassing process. For this reason, a material with the specific surface area of 10 m2/g or less is used as the lower film. With this, adsorbed moisture and carbon dioxide can be easily eliminated through the heat degassing process at 350xc2x0 C or less. Even if the heat degassing process is insufficient, the residual amount adsorbed in the protective film is less. The time required for the heat degassing process depends upon the panel size and cell structure and also on the capacity and method of the degassing system. Thus, the time cannot always be determined in a simple manner but is two hours or so approximately for normal panels.
An oxide is used as the protective film for the PDP according to the present invention, and an oxide film mainly made from magnesium oxide is particularly preferred. Naturally, any other components may be contained so as to change the characteristic of the magnesium oxide.